Many musical instruments include strings which the player imparts movement to generate sound. In the case of an electric guitar, the audio sound is produced indirectly from the motion of the string, typically steel strings, over a magnetic pickup. The magnetic pickup includes a bobbin wrapped with a coil of fine enameled copper wire and a permanent magnet with a core material, such as alnico or ferrite. The pickup is most often mounted on the body of the guitar. The permanent magnet creates a magnetic field, and the strings are routed through the magnetic field. When the electric guitar is played, the motion of the vibrating steel strings disturbs the magnetic field and changes the magnetic flux to induce a voltage in the coil. The changes in voltage from the vibrating strings produce a current in the coil representative of the string motion and, correspondingly, the sound intended by the player. Accordingly, the pickup operates as a transducer or variable reluctance sensor that converts mechanical string vibrations to an electrical signal, which is routed to an amplifier and loudspeaker to reproduce an audible sound.
In a traditional single coil magnetic pickup, the turns of wire in proximity to each other have an equivalent self-capacitance that, when added to cable capacitance, resonates with the inductance of the winding. The resonance accentuates certain frequencies, giving the pickup a characteristic tonal quality. More turns of wire in the winding produces a higher output voltage and lower resonance frequency. The inductive source impedance inherent in a single coil pickup makes it less linear than other forms of pickups, such as piezo-electric or optical transducers. The tonal quality produced by such nonlinearity is, however, subject to taste, and some artists consider the sound aesthetically superior to a more linear transducer.
The single coil pickup is susceptibility to external electromagnetic interference (mains hum), e.g., from electrical power cables, power transformers, and fluorescent light ballasts in the area, as well as the magnetic interference with the natural vibrations of the strings. Mains hum typically includes a fundamental frequency at 50-60 Hz and some harmonic content. The changing magnetic flux caused by the mains current links with the windings and induces an undesired voltage variation in the pickup.
FIG. 1 shows a typical humbucking pickup 10 for overcoming the mains hum effect by using two coils 12 and 14, with each coil wound or connected in opposition to the other around bobbin 16. A plurality of cylindrical permanent magnets 20 are disposed through bobbin 16 in an interior area of coil 12 in humbucking pickup 10, and a plurality of cylindrical permanent magnets 22 is disposed through bobbin 16 in an interior area of coil 14. Magnets 20 are aligned with magnets 22 in the respective windings. Since ambient hum from power-supply transformers, radio frequencies, or electrical devices reaches the coils as common-mode noise, the external interference induces an equal and opposite voltage in each coil, which tends to cancel each other out due to the opposition of coils 12 and 14.
In particular, humbucking pickup 10 is mounted to electric guitar body 30 with cylindrical magnets 20 and 22 oriented perpendicular to guitar strings 32, as shown in FIG. 1. Magnet flux lines emanating from the north pole N of permanent magnet 20 are perpendicular or normal to the longitudinal axis of strings 32. Strings 32 vibrate through the magnetic field across the north pole N of cylindrical magnets 20, which changes the magnetic flux and induces a voltage in coils 12 and 14. The changes in voltage from the vibrating string 32 produces a current in coils 12 and 14 representative of the string motion and, correspondingly, the sound intended by the player. The electrical signal is routed to an amplifier and loudspeaker to reproduce an audible sound.
Coils 12 and 14 are wound or connected in opposition so that external interference, e.g., ambient hum from power-supply transformers, radio frequencies, or electrical devices, induces an equal and opposite voltage in each coil which tends to cancel out external interference. Humbucking pickup 10 with permanent magnets 20 and 22 oriented perpendicular to guitar strings 32 achieves a level interference cancellation, albeit with the associated higher cost and larger form factor.